This invention relates to a treatment furnace for heating workpieces in an atmosphere of controlled composition. For example, the invention may be applied to a carburising furnace intended for heating metal workpieces in a carbon-rich atmosphere to carburise the workpieces and to a furnace intended for heating workpieces which are to be hardened in an atmosphere having a carbon potential matching the surface composition of the workpieces.
At the present time, mild steel workpieces are carburised to form a hard case at the outer surface of the workpieces by heating the workpieces in contact with a cyanide compound, in contact with elemental carbon, or in contact with a carbon-rich atmosphere.
The use of solid elemental carbon is not popular, since the process is slow and messy. The cyanide method has the advantage that low capital costs are incurred, but the process gives rise to serious problems concerning disposal of the used cyanide compound and safety of the operatives. Carburising by means of an atmosphere-rich in carbon (gas carburisation) can be carried out relatively quickly and conveniently and can conveniently be carried out as a continuous process rather than a batch process. However, gas carburisation requires the use of a furnace which is arranged to prevent ingress of the ambient atmosphere and also the use of a generator for producing the carbon-rich atmosphere required in the furnace. This equipment is very expensive, compared with that used in the alternative processes and requires a considerable capital investment. There is therefore a requirement for a less expensive apparatus in which gas carburising can conveniently be performed.